Process for making paper and air-pervious cardboard or boardlike structures predominantly of polytetrafluoroethylene



United States Patent 3,528,879 PROCESS FOR MAKING PAPER AND AIR- PERVIOUS CARDBOARD OR BOARD- LIKE STRUCTURES PREDOMINANTLY OF POLYTETRAFLUOROETHYLENE Yutaka Kometani, Hyogo-ken, Shun Koizumi, Osaka, and Kazuo Kubota and Takeaki Nakazima, Osaka-fu, Japan, assignors to Daikin Kogyo Co., Ltd., Osaka, Japan, a corporation of Japan No Drawing. Filed Oct. 12, 1964, Ser. No. 403,367 Claims priority, application Japan, Oct. 14, 1963, 38/5 3,483; Nov. 13, 1963, 38/60,627; June 11, 1964, 39/32,995; June 17, 1964, 39/33,932

Int. Cl. D21f 11/00; D21h /12 U.S. Cl. 162157 5 Claims ABSTRACT OF THE DISCLOSURE A process for producing paperlike or boardlike airpervious sheets and such air-pervious sheets so produced which process comprises dispersing a polytetrafiuoroethylene fibrous powder having an average fiber length of 100 to 5000 microns, an average shape factor of not less than 10, and an anisotropic expansion factor of 1.30 to 7.00 in a liquid whose surface tension at C. is below dynes per centimeter with subsequent formation of a web from such dispersion and sintering thereof. The dispersion may also contain a thermoplastic resin powder such as selected from the group consisting of polytetrafiuoroethylene particles and tetrafluoroethylene-perfiuoroolefin copolymer particles.

This invention relates to a process for making strong, air-pervious sheets, i.e., paper having a thinness of less than 200 grams per square meter, and pliable cardboard or boardlike structures, consisting of polytetrafiuoroethylene or polytetrafiuoroethylene with other additives, e.g., thermoplastic resins, glass fiber, asbestos, etc. More particularly, the invention relates to a process for making from polytetrafiuoroethylene fibrous powder having an average fiber length of 100-5000 microns, an average shape factor of not less than 10 and an anisotropic expansion factor of l.307.00, a strong, air-pervious sheets, which are suitable as filter material.

The present invention further relates to paper and airpervious cardboard or boardlike structures predominantly of polytetrafiuoroethylene, which have been reinforced with an air-pervious reinforcing material layer; and to a process for making the same.

Although a number of inventions have been made concerning the process for producing filter material of polytetrafiuoroethylene, each has its drawbacks and none has come to be widely practiced.

Of the prior art processes, that being practiced most frequently is the process in which polytetrafluoroethylene is mixed with either crystalline powder of salt or metallic powders of copper, iron, aluminum, etc., the mixture then preformed by compressing in a metallic mold, followed by sintering the preform at above 327 C. and thereafter dissolving and removing the admixed salt or metallic powders. Another process is that in which naphthalene powder is mixed in, then after vaporizing the mixture at below the melting point of the resin, sintering is carried out. The molding step in either of these processes is complicated, and not only is there the drawback that the additives are not completely eliminated but they also have the defect that their strength, especially fiexal strength, is small. Further, because of the complexity of their making step, they, of necessity, become expensive. It is also known to make filter cloth by weaving into a fabric the polytetrafluoroethylene fiber obtained by the process disclosed in 3,528,879 Patented Sept. 15, 1970 US. Pat. 2,772,444. A filter cloth such as this is very expensive however and can only be used for special purposes.

Another known process is that in which the colloidal particles of polytetrafiuoroethylene to which has been added a lubricant are extruded from a fine nozzle to produce rods or tubes, following which these are cut into pieces 6-25 mm. long, then applied an abrasive force to render them fibrous, after which this fibrous product is pulped by placing in water or water to which a surfactant has been added and thereafter made into a paper product (US. Pat. 3,003,912). In this process however, it should be self-evident that thin papers cannot be made, since the length of the fibers used is long. Even when thicker sheets are to be made, it is necessary to run the sheets through a calender roll several times to remove the nonuniformity in the thickness of the sheets. At times, however, an unnecessary amount of interlacement between the fibers occurs to result in the density becoming too excessive, with the consequence that air-permeability is lost. In addition, since the so-called paste extrusion is carried out during the course of producing the fibers, this results in setting up in the fibers molecular orientation to a high degree, which orientation is relaxed at above the melting point to cause undesirable shrinkage to take place during the sintering step. This shrinkage not only increases the thickness of the paper but becomes the cause of nonuniformity in thickness as well. Further, since extrusion is carried out during the fiber-making step according to this process, it becomes economically an expensive filter material.

Strong paper of a thinness less than 200 grams per square meter and air-pervious, pliable cardboard or boardlike structures of polytetrafiuoroethylene could not be made economically to advantage by these prior art processes.

Further, a filter material predominantly of polytetrafluoroethylene, which has good properties and is of low cost, was not known.

As a result of extensive experimentation, we found that the thin paper and cardboard or boardlike structures of polytetrafluoroethylene, as contemplated by the present invention, could only be made by using as the starting material the polytetrafiuoroethylene fibrous powder having an average fiber length of 100-5000 microns, an average shape factor of not less than 10 and an anisotropic expansion factor of 1.30-7.00; and further that this special fibrous powder could be made into paper and cardboard or boardlike structures by only the process hereinafter described.

The terms average fiber length and average shape factor, as used herein, denote respectively the arithmetical average of the lengths of 200 or more fibers observed optically when said powder is examined under a microscope and an arithmetical average of 200 or more shape factors which is obtained by dividing the fiber length by its width. In measuring the fiber lengths from the photomicrographs, those of fiber length not exceeding microns must not be measured. Since the average according to this measurement method is a number average, should those fibers not exceeding 80 microns, the proportion by weight of which are very small, are added to the number average, it will result in evaluating unjustifiably low the fiber length obtained, thus becoming a value far different from the actual fiber length.

The term anisotropic expansion factor is determined by the following method: Four and one-tenth grams of powder is weighed into a 0.5 inch square metallic mold where it is subjected at 23 C. to a pressure raised to 2000 p.s.i. during one minute, after which it is held at this pressure for two minutes. The length, width and height of the resulting roughly cubical preform are measured (i.e., the X, Y and Z axes, respectively, where Z axis is the direction in which the preforming pressure was applied). The measured preform is sintered for 30 minutes at 380 C.:L0.5 C., followed by allowing the resulting sintered product to cool in air to room temperature, after which it is remeasured. The anisotropic expansion factor is then the value of Z /Z divided by where X Y Z are the respective axial measurements of the preformed product, while X Y and Z are the axial measurements of the sintered product.

Unless the particles are fibrous, the interlacement that occurs between fibers do not take place and hence good quality paper cannot be made. Namely, the powder which heretofore was readily available commercially was in nearly all cases not fibrous, with the consequence that paper and cardboards could not be made. Since the average shape factor is a factor which numerically expresses the extent of the fibrous state, the interlacement does not occur fully substantially when this factor is less than 10.

However, even though the powder may be of fibrous form, if its fiber length is less than 100 microns, the length of the fiber is too short and hence in this case also it cannot be made into paper, as the interlacement between the fibers is insufficient. As commercially available powders of this grade, there is one which has an average particle diameter of 35 microns, a shape factor of 8-12 and an anisotropic expansion factor of 1.18. As a practical matter, a paper cannot be made from this powder, however. While this is due to the fact that its shape factor is small and the extent of its fibrousness is small, there is also the following-reason. Namely, since average fiber length of this powder is less than 100 microns, when paper is made from its dispersion, it does not become paperlike because of the lack of interlacements between the fibers.

On the other hand, when the powder has an average fiber length exceeding 5000 microns, it is not suited for making thin but strong paper and cardboards that are uniform, even though the other properties, including the shape factor and anisotropic expansion factor, are within the limits prescribed by this invention. Namely, when it exceeds S000 microns, the irregularity in the surface of paper made therefrom is pronounced. That is to say, when the fiber length is great, the diameter of the fibers becomes large, thus making it impossible to make thin and uniform papers. As a process for producing this type of fiber, there is one such as disclosed in the previously mentioned U.S. Pat. 3,003,912. However, while the fibers obtained by this process can, with some difficulty entailing, be formed into air-pervious cardboards and sheets, they could not be made uniform since the fiber length of the powder was greater than 5000 microns, and hence there were spottiness in the strength of the products obtained. Further, it was not possible at all to make thin papers of less than 200 grams per square meter.

Further, the powder becomes unfit for making uniform thin papers, cardboards and sheets when its anisotropic expansion factor is not less than 7.00, even though its other properties fall within those prescribed by the invention. Namely, the anisotropic expansion factor is a measure of the molecular orientation in the fiber. Hence, as the molecular orientation increases the anisotropic expansion factor becomes greater. If the molecular orientation is great, the shrinkage of the fiber at above the melting point of polytetrafluoroethylene is inevitably great, and in consequence spottiness in the thickness of the product results because of the great shrinkage in the paper during its sintering step.

When the anisotropic expansion factor is less than 1.30, the form of the powder is either nonfibrous or not completely fibrous, and thus since interlacement does not take place between the particles of the powder, it cannot be made into paper.

According to the process of the present invention, a polytetrafluoroethylene fibrous powder having an average fiber length of 1004000 microns, an average shape factor of not less than 10 and an anisotropic expansion factor of -700 is weighted, to which is then added on a weight basis fivefold or more of a liquid whose surface tension at 25 C. is below 40 dynes percentimeter, and preferably below 35 dynes per centimeter. Any liquid so long as its surface tension is below 35 dynes per centimeter will do. Preferably employed are, for example, aliphatic hydrocarbons such as hexane, heptane, gasoline and kerosene, or the mixtures thereof; aromatic hydrocarbons such as benzene, toluene and xylene; alcohols such as ethyl alcohol, methyl alcohol, isopropanol, tertiary butanol, allyl alcohol, ethylene glycol, benzyl alcohol and cycohexanol; others such as ethyl ether, anisol, tetrahydrofuran and dioxane; aldehydes such as paraaldehyde, acetal and acrolein; ketones such as acetone, cyclohexanone and methyl ethyl ketone; halogen derivatives such as chloroform, carbon tetrachloride, allyl iodide, ethylene dibromide, chloral, dichloroacetic acid, acetyl chloride, monochlorobenzene and benzyl chloride; fiuoro-derivative such as trichlorotrifiuoroethane, 'monofluorotrichloromethane, difluorotetrachloroethane, octafiuorocyclobutane,

H(OF CF ),,CH OH, Cl(CF CF Cl, where n in the foregoing three formulas is an integer 1 to 10, omegamonohydroperfluorohexene, benzotrifluoride, monobenzotrifluoride, dibromotetrafluoroethane, and trichloropentafluoropropane, and the mixtures thereof. Besides these, water whose surface tension has been lowered to below 40 dynes per centimeter, and preferably below 35 dynes per centimeter, by the addition of a suitable surfactant, say, such as alkyl-aryl-ethylene glycol, can be used.

The reason why the surface tension must be below 40 dynes per centimeter, and preferably below 35 dynes per centimeter, is because the specific surface area of the fibrous powder used in this invention is ten times that of the fibers according to the conventional processes. For example, the specific surface area, as obtained by nitrogen adsorption, of the fibers used in making the two hereinbefore described conventional filter materials is 0.05- 1.0 m. /g., whereas that of the present invention is 0.5-3 m. g. Hence, since the invention powder is more susceptible as a whole to surface tension than the other fibers, it is difficulty wetted with liquids. Therefore, if a liquid of great surface tension, such as water, is used, the fibers would float and it would be impossible to form a uniform paper Web.

We also discovered that of these liquids, the fluoroderivatives such as trichlorotrifiuoroethane, difluorotetrachloroethane and monochlorobenzotrifluoride were particularly to be preferred as dispersion medium. Not only because these fluoro-derivatives have low surface tension but also because they are compatible with polytetrafluoroethylene, the powder disperses well, there being practically no formation of lumps of the powder. Hence, uniform papers can be made. In addition, when these fluoro-derivatives have been used as the dispersion medium, the powder dispersed is characterized in that the dispersion is stable and can be kept for long periods of time without the formation of lumps.

While methyl alcohol, ethyl alcohol and acetone and the aqueous solutions as well as carbon tetrachloride also are good dispersion mediums, they have a slight tendency to the formation of lumps in the powdered fiber when compared with the fluoro-derivatives.

When using water as the dispersion medium by lowering its surface tension by adding a surfactant thereto, it was found that instead of using it as such, still better results could be obtained by using it mixed with 05-40% by weight of either polytetrafluoroethylene particles or tetrafluoroethylene-perfluoroolefin (CF =CFR where R is a radical consisting of a carbon atom and fluorine atom) copolymeric particles whose average particle diameter is 0.05-0.5 micron, or the mixture thereof, as hereinafter described.

In making paper using the so obtained dispersion, the conventional paper making machines, particularly the paper making machines which are manually operated such as the TAPPI type can be used without change or with only minor modifications.

The paper web which has been merely dried cannot be used in this state, since the bonding between the fibers is not sufliciently strong. But by heating this dried web, a paper product having sufficient strength can be obtained. As there is a tendency to shrinkage occuring in the fibers as the sintering temperature becomes higher, a temperature of below 360 C. is preferably employed.

As hereinbefore described, in making paper and airpervious cardboard or boardlike structures, predominantly of polytetrafiuoroethylene, although basically the process comprises dispersing a polytetrafiuoroethylene fibrous powder having an average fiber length of 100-5000 microns, an average shape factor of not less than and an anisotropic expansion factor of 1.30-7.00, in a liquid of below 40 dynes per centimeter, then forming a web using this dispersion and thereafter heating the formed web with or without application of pressure, the present invention comprehends several more valuable embodiments. For example, paper and air-pervious cardboard or boardlike structures predominantly of polytetrafiuoroethylene can be made by dispersing a polytetrafiuoroethylene fibrous powder having an average fiber length of 100-5000 microns, an average shape factor of not less than 10 and an anisotropic expansion factor of 1.30- 7.00, and 02-50% by weight of a thermoplastic resin powder, in a liquid having a surface tension at C. of below 40 dynes per centimeter, and preferably below dynes per centimeter, then after forming a web using this dispersion, either heating the web formed to above the melting or softening point of the thermoplastic resin incorporated or heating the web after pressing or hot pressing the web.

The thermoplastic resin preferred for dispersing with the polytetrafiuoroethylene fibrous powder include the polytetrafiuoroethylene powders having a wet-sieve size of less than 500 microns, tetrafluoroethylene-hexafiuoropropylene copolymer powder, polyethylene powder, polyvinyl chloride powder, polypropylene powder, trichlorotrifluoroethylene powder, polyacetal powder, the various polyamide resin powders, tetrafiuoroethylene-vinylidene fluoride copolymer powder, polyvinylidene chloride powder, vinyl chloride-vinylidene chloride copolymer powder, Y

ethylenepropylene copolymer powder, or the particles of polytetrafiuoroethylene and/or copolymers of tetrafluoroethylene-perfiuoroolefin whose particle diameter is 0.05- 0.5 micron, as hereinbefore noted. Particularly, when the polytetrafiuoroethylene powder of a wet-sieve size less than 500 microns or the tetrafluoroethylenehexafluoropropylene copolymer powder is used, since it is possible to make paper and cardboard or boardlike structures having great strength without impairing whatsoever the various properties of the predominant polytetrafiuoroethylene, such as its excellent heat resistance and resistance to chemicals, the use of these powders give especially remarkable results. On the other hand, in the case of thin paper and airpervious cardboard or boardlike structures predominantly of polytetrafiuoroethylene for use in making articles not requiring heat resistance and resistance to chemicals, for example, speaker cones, the conjoint use of a resin of low melting or softening point makes possible the production of boardlike structures economically.

While, as the polytetrafiuoroethylene powder to be conjointly used, any will do so long as it is a powder of a wet-sieve size less than 500 microns, particularly preferred are such as the finely divided powders obtained by grinding in the Ultramizer (a product of Fuji Electric Works Co. Ltd., Japan) either the fine powder obtained by coagulating an aqueous dispersion of colloidal polytetrafiuoroethylene obtained by emulsion polymizeration, the commercial grade polytetrafiuoroethylene powder, or the powder obtained by the polymerization in the vapor phase of tetrafiuoroethylene at 3-10 atmospheres and 0-40" C. in the presence of water containing a reaction initiator of free radicals; or the finely divided powder obtained by grinding the commercial grade polytetrafiuoroethylene molding powder (normally of particle diameter 300-1000 microns) in the Micron Mill (product of Hosokawa Iron Works Ltd., Japan) or Jet-O-Mizer.

While it may seem strange that the strength is increased when, as in this case, granular powder of the same type of substance is added and mixed with polytetrafiuoroethylene fibrous powder, this fact has been made known for the first time by means of this invention.

The fibrous powder having an average fiber length of -5000 microns, an average shape factor of not less than 10 and an anisotropic expansion factor of 1.30-7.00 differs in its melting temperature of the crystals and shrinkage temperature of fibers, the latter being higher. The melting point of the polytetrafiuoroethylene granular powder is the same as that of the melting temperature of the crystals of fibrous powder, and since the powder becomes completely gelled at above the melting point, the intimate bonding between the fibrous powder is effected to increase the strength of the product if it is sintered above the melting temperature of the crystal but at a temperature which does not change the form of the fibrous powder. Further, since the melting point of the tetrafluoroethylene-hexafiuoropropylene copolymer is lower than that of polytetrafiuoroethylene, it becomes a good binder, with the consequence that the strength of the molded product is enhanced. Thus, even the fibrous powder having an anisotropic expansion factor of above 1.30 and below 7.00, which could not have been molded by the prior art processes unless much care was exercised in its sintering can now be readily molded into thin paper and cardboard or boardlike structures by its conjoint use with the various thermoplastic resin powders, and particularly with the polytetrafiuoroethylene powder having an average particle diameter below 500 microns or the tetrafluoroethylene-hexafiuoropropylene copolymer powder.

While the strength of the resulting paper or cardboard and boardlike structure will be increased in concomitance with an increase in the amount of the conjointly used powder, the air-permeability or the filtration speed is de creased. Thus, the conjoint use of a powder such as, for example, of polytetrafiuoroethylene or the tetrafiuoroethylene-hexafluoropropylene copolymer plays the role of a regulator of air-permeability.

It accordingly becomes necessary to increase or decrease the amount of the conjointly used powder depending upon the purpose to which the resulting molded product is to be put. When a paper of great air-permeability or filtration speed is required, the conjointly used powder in an amount of 1-15 by weight is suitable but, on the other hand, when one is required which is strong though its air-permeability is small, the amount suitably used is 15-50% by weight. An amount ranging between 0.2% and 50% by weight is chosen depending upon the strength and air-permeability desired in the product.

The method of making conjoint use of a powder may be that in which the powder to be conjointly used is added after the fibrous powder has been dispersed in the dispersion medium or that in which the fibrous powder and the powder to be conjointly used are mixed in a dry type mixer, etc.

We also found that good quality paper and air-pervious cardboard or boardlike structures predominantly of polytetraflouorethlyene could be made by dispersing a fibrous powder having an average fiber length of 100- 5000 microns, an average shape factor of not less than 10 and an anisotropic expansion factor of 1.30-7.00, in a dispersant containing -40% by weight of particles of polytetra-fiuoroethylene and/ or a tetrafluoroethyleneperfluoroolefin copolymers (CF =CFR where R is a radical consisting of carbon and fluorine) whose average particle diameter is 0.05-0.5 micron, then using this dispersion to form a web, and thereafter heating the web at above 270 C. under atmospheric or superatmospheric pressures.

The particles used in this instance consisting of that of polytetrafiuoroethylene or a copolymer of tetrafluoroethylene and perfluoroolefin (CR =CFR where R is a perfiuoroalkyl radical) of which the average particle diameter is below 0.5 micron can be obtained readily as an aqueous, colloidal dispersion by the emulsion polymerization according to customary procedures of tetrafluoroethylene alone or a mixture in optional proportions of tetrafluoroethylene and perfluoroolefin in the presence of water, using an emulsifier and a polymerization initiator of free radicals. Further, conveniently useable are also the commercial grade polytetrafluoroethylene dispersion or the fluoroethylene-propylene copolymer dispersion in which C F has been used as a copolymer constituent. These dispersions normally contain several percent of a surfactant and thus can be used, as such, for making the papers as contemplated by this invention, but since their content of resin is great and as a result they yield products lacking in pores, they are used diluted With water or an ion-exchanged water.

The 0.05-0.5 micron polytetrafluoroethylene or tetrafiuoroethylene-perfluoroolefin copolymer contained in the aqueous dispersion used in the invention can be used in any concentration provided it is between 0.5% and 40% by weight. Naturally, the paper and cardboard or boardlike structures obtained by forming the fibrous powder into a Web using a dispersion containing a great amount of resin will become relatively dense and the surface of such a paper, cardboard and boardlike structures will be smooth and have a good touch. Conversely, when the Web is formed using a dispersion whose content of resin is small, the product obtained will be one having great airpermeability. Hence, the choice of the concentration of dispersion will be decided in accordance with the use to which the final product is to be put.

The addition in this manner of a small amount of the aforesaid dispersion in the dispersion in which the fibrous powder has been dispersed has the effect of enhancing the luster of the surface of the paper or cardboard and of also increasing their strength somewhat. Thus, it becomes possible either to omit the step or reduce the time required for finishing the product by means of rolls following the formation of the web and heating.

Another merit in connection with the use of the dispersion of 0.05-0.05 micron polytetrafluoroethylene and/ or tetrafluoroethylene-perfluoroolefin copolymer is that when this dispersion medium to which has been dispersed the fibrous powder having an average fiber length of 100-5000 microns, an average shape factor of not less than 10 an anisotropic expansion factor of 1.30-7.00 is used and is formed into a web above a liquid-pervious reinforcing material layer which can retain its original form even though heated to 270-380 C., and thereafter the web formed is heated along with the reinforcing material layer at a temperautre of 270-3 80 C. under atmospheric or superatmospheric pressure, the production of reinforced paper and liquid-pervious cardboard or boardlike structures predominantly of polytetrafluoroethylene is possible.

The paper and liquid-pervious cardboard or boardlike structures predominantly of polytetrafiuoroethylene obtained from the fibrous powder thereof having an average fiber length of 100-5000 microns, an average shape factor of not less than 10 and an anisotropic expansion factor of 1.30-7.00. alone or in a state in which it contains a small amount of a thermoplastic resin powder by forming into a paper web and sintering exhibit excellent performance as filter material at low pressures, but when used at high pressures, for example, in separating water contained in gasoline, they encounter trouble with respect to their bursting strength.

Now, however, if in accordance with the invention, the Web is formed on top of a liquid-pervious reinforcing material layer which can retain its original form even though heated to 270-380" C., such as, for example, a wire screen, metallic wool, glass cloth and glass wool, this defect of the conventional process can be overcome and thus it is possible to provide improved thin paper and liquid-pervious cardboard and boardlike structures predominantly of polytetrafluoroethylene, which can be used even under high pressures.

Although the 0.05-O.5 micron polytetrafluoroethylene and/or tetrafluoroethylene-perfiuoroolefin copolymer contained in the dispersion may be in any concentration within the range of 0.5% to 40% by weight, an aqueous dispersion containing a greater amount of resin enhances the adhesion of the paper and liquid-pervious cardboard or boardlike structures to the reinforcing material layer. Liquid-permeability, however, declines as the content of the resin increases in the aqueous dispersion. And when an aqueous dispersion containing more than 40% by weight of the particles of polytetrafluoroethylene and/or tetrafluoroethylene-perfluoroolefin copolymer whose average particle diameter is 0.05-0.5 micron is used, the paper and cardboard or boardlike structures will adhere perfectly to the reinforcing material layer, but will become such that it is practically without liquid-permeability, thus rendering it impossible to attain the objects of the present invention.

On the other hand, when the content of resin in the aqueous dispersion is less than 0.5% by weight, the paper and cardboard or boardlike structures will not adhere to the reinforcing material layer at all, or at most, imperfectly. Thus the objects of the invention cannot be achieved.

The following are examples to further illustrate the present invention.

EXAMPLE 1 150 cc. of trichlorotrifluoroethane was added to 3 grams of polytetrafluoroethylene fibrous-powder whose average fiber length was 400 micron, and average shape factor was 27, anisotropic expansion factor was 1.62 in a flask, and the mixture was well shaken up so as to leave no lump of powder and a vessel of which diameter was about 21 cm. was charged with about 500 cc. of trichloroethane and a sieve of mesh with a diameter of mm. was sunk therein.

As to the amount of trichlorotrifluoroethane in a vessel, it is sufiicient if the net of the sieve can be dipped.

When the above dispersion substance thus prepared was transferred into the sieve, the powder spread all over the net of the sieve uniformly.

Few minutes later the sieve was quietly taken up, and was dried. When the web of polytetrafluoroethylene fibrous powder on the sieve was sintered for 30 minutes in an electric furnace at 335 C., a uniform thin paper of polytetrafluoroethylene was obtained.

The characteristics of the obtained thin paper are given in Table I as the sample Number A. There was observed no shrinkage of the paper while sintering.

EXAMPLE 2.

By employing 6 grams of fibrous powder and another 10 grams of fibrous powder of the kind used in Example I, paper was produced by the same operation as in Example 1.

The characteristics of the obtained papers are given in Table l as B and C.

EXAMPLE 3 30 grams of polytetrafiuoroethylene fibrous powder having an average fiber length of 850 microns, average shape factor of 30, anisotropic expansion factor of 1.75 was added to 300 cc. of carbon tetrachloride, and the mixture was well stirred to produce a dispersion.

A piece of filter paper of the same diameter was placed in the sieve of which the diameter was 144 mm., and the powder was spread in the form of web by the same operation as in Example 1.

This spread powder was taken out with the filter-paper from the sieve, and the pressure of kg. per square centimeter was applied. Thereafter the filter paper was taken away and sintered in a furnace at 340 C. for 40 minutes. The obtained thin sheet was flexible, air permeable and very strong.

The characteristics thereof are given in the following Table 1 as D.

EXAMPLE 4 The paper obtained in Example 2 whose sample number is B was rolled twice at about 100 C. As a result, the permeability was a little bit lowered but the strength became higher.

The characteristics thereof are given in Table 1 as E.

10 1.5 kg./ mm., and the strength in water was almost the same in the air. The water permeability was 13 seconds, and the paper could be used as a filter.

EXAMPLE 7 EXAMPLE 8 1 part of polytetrafluoroethylene fibrous powder having an average fiber length of 950 microns, an average shape factor of 38, and anisotropic expansion factor of 5.2, was mixed with 200 parts of polytetrafluoroethylene aqueous dispersion.

A dispersion was prepared by diluting polytetrafluoroethylene dispersion containing about 60% by weight of a TABLE 1.CHARACTERISTICS OF PAPERS, CARDBOARDS, AND BOARDLIKE STRUCTURES OF POLYTETRA- FLUOROETHYLENE Retention properties of precipitates 1 Water Air Tensil strength 3 permeperme- Ferric kg./15mm. Elongation (percent) Sample Basis weight Thickness ability 1 ability 2 hydro Lead Barium N0. (g./m. (mm.) (see) (Llem. min.) oxide sulfate sulfate In air In air In water In water 183 0.2 12 Good Good Good 0. 53 0.50 37 37 366 0. 6 0 l. 0 1. 0 33 33 612 1. 0 1. 38 1. 28 33 33 1, 836 2. 0 7.0 7. 0 34 34 355 0. 4 1. 5 1. 5 30 1 According to the test method of I IS P 3801 (1956).

EXAMPLE 5 2.7 grams of polytetrafiuoroethylene fibrous powder having an average fiber length of 950 microns, an average shape factor of 38, an anisotropic expansion factor of 5.2 was mixed with 0.3 g. of fine powder obtained by grinding powder of commercial grade polytetrafluoroethylene with Ultramizer of Fuji Electric Works Co., Ltd. and 150 cc. of trichlorotriflnoroethane was added thereto to obtain a dispersion. About 500 cc. of trichlorotriflnoroethane was put in a vessel with a diameter of about 21 cm., and a 100 mesh sieve made of stainless steel with a diameter of 140 mm. was sunk therein.

As to the amount of trichlorotriflnoroethane in the vessel, it is sufficient if the net of the sieve is dipped therein.

The dispersion having been already prepared was transferred into the sieve, and the powder was uniformly spread on the net of the sieve.

Thereafter, the sieve was quietly pulled up, dried and then sintered for 30 minutes at 340 C. As a result, thin paper of 180 grams per square meter of polytetrafluoroethylene was obtained.

There was observed no shrinkage in the paper during the sintering.

The tensile strength of the paper was 1.5 kg./ 15 mm. and the strength in the water was almost the same as in the air.

The water permeability was 13 seconds, retension properties of precipitates were excellent, and it could be used as the fine filter paper. In case where the fine powder was not added to fibrous powder, the strength of the paper of the same thickness obtained was 0.53 kg./ 15 mm.

EXAMPLE 6 0.3 gram of tetrafluoroethylene-hexafluoropropylene copolymer powder (hexafluoropropylene is contained therein in an amount of 17% by weight) was added to 3 grams of the fibrous powder which was used in Example 1, and according to the same process as that of Example 1, paper was produced. The tensile strength of obtained paper was commercial grade resin with ion exchange water to such a degree that the resin content became 10%. And to the said dispersion, the surface active agents of polyoxyethyleue, alkylallylether type and alkyl betaine type were respectively added by about 0.3%. The surface tension of dispersion at 25 C. was 35 dyne/ cm.

The said dispersion containing fibrous powder was treated with a TAPPI type paper machine having a diameter of 230 mm., and then it was peeled off from the wire net, and was dried, and then sintered at 335 C. in an electric furnace for 20' minutes.

The obtained paper of polytetrafluoroethylene was 180 g./m. thick, and the tensile strength thereof was 18 kg./ 15 mm. The water permeability was 23-25 seconds, and retention properties of precipitates were excellent. The surface of the paper was uniform and there was no uneveness.

EXAMPLE 9 By employing the same operation as in Example 8, (10% of resin contained in the dispersion was tetrafluoroethylene-hexafluoropropylene to 15) copolymer) the surface tension of the dispersion at 25 C. was adjusted to 37 dyne/cm. and the same surface active agents as employed in Example 8 were used.

The tensile strength of thus obtained paper was almost the same, namely 1.8 kg./ 15 mm., either in air or in water.

The water permeability was 20 seconds.

The surfaces of the papers obtained in Examples 8 and 9 proved smoother than those obtained in each of the examples up to Example 6.

EXAMPLE 10 In Example 8 the amount of polytetrafluoroethylene contained in the dispersion was adjusted to 15%, and other operations were so adjusted to be equal to those of Example 8, and thereby the dispersion containing fibrous powder was prepared. In subjecting the said dispersion to the TAPPI type paper machine with a diameter of 230 mm., the dispersion was screened on the carefully placed 200 mesh stainless steel wire net so as not to cause a wrinkle on the wire net of the said paper machine, and almost all the fibrous powder was spread on the stainless steel wire net uniformly, and no powder was recognized on the wire net of the paper machine.

Then the stainless steel wire net was dried with the fibrous powder spread over the net, and when they were thereafter sintered at 335 C. for 20 minutes, there was obtained paper whose main component was polytetrafluoroethylene which was completely melt-adhered to the net.

The water permeability thereof was 80 seconds, and retention properties of precipitates were excellent, and the surface thereof was smooth and uniform.

EXAMPLE 11 This example is almost the same as Example 10, but in this example tetrafluoroethylene-hexafluoropropylene (85 to 15) copolymer dispersion was employed, and the resin content was adjusted to 20%, A, the surface tension of the dispersion at 25 C. Was adjusted to 37 dyne/ cm. by employing the same surface active agent as in Example 10. Instead of the stainless steel wire-net used in Example 10, glass cloth was used, and the paper of polytetrafluoroethylene completely melt-stuck to the said glass cloth was obtained.

CONTROL The paper whose main component was polytetrafluoroethylene reinforced by the wire-net as was obtained in Example was fixed on the bottom of stainless steel cylindrical vessel so as not to have the liquid leak out from the contact surface of the paper and the cylindrical vessel. Then the cylindrical vessel with the paper was set perpendicularly, and a glass receiver was provided at the lower part of the cylindrical vessel.

Then, when the mixture of 50 parts of carbon tetrachloride and 50 parts of Water was poured from above the said cylindrical vessel, carbon tetrachloride alone passed through the paper whose main component was polytetrafluoroethylene, but Water did not pass through.

The similar tests were conducted against gasolineaqueous mixture, trichlorotrifiuomethane-aqueous mixture and in either case water stayed at the upper part of the paper, whose main component was polytetrafluoroethylene, and did not pass through the filter itself.

The aqueous portion in the solution received in the glass receiver at bottom after passing through the paper was quantitatively analyzed by Karl Fisher method, and the results as given in Table 2 were obtained.

TABLE 2 Aqueous portion Aqueous portion Thus obtained papers, cardboards, and boardlike structures can be used as nonfiammable excellent filter material which is free from corrosion by any chemicals. Polytetrafluoroethylene is originally water repelling, and this filter paper hardly allows water to pass, but in case water and aqueous solution are filtered, the said filter material is saturated with such hydrophillic solutions as to wet well polytetrafluoroethylene such as methanol, ethanol, and acetone, and then the said solution is replaced with water, and by so doing, we found the filter material passes water.

Therefore, the filter paper obtained according to the present invention can be used as the conventional filter papers almost in the same manner, and the tensile strength in the water is almost the same as in the air, and when the precipitate is scraped the said filter paper does not break, nor the fiber thereof goes off.

The filter paper, =after having been used, is washed with such solutions as to dissolve the precipitate, and is dried up, and the used filter paper can restore the original state, and it has such excellent property as can stand repeated uses.

The thin polytetrafluoroethylene paper obtained according to the present invention is very soft, and can be used as a substitute for deerskin for cleaning lenses and precision machines and tools, and since the tensile strength thereof is high, the usages thereof as non-woven cloth can be considered.

What we claim is:

1. A reinforced air-pervious sheet predominantly of polytetrafluoroethylene, characterized in that said sheet consisting predominantly of a polytetrafluoroethylene fibrous powder having an average fiber length of to 5000 microns, an average shape factor of not less than 10, and an anisotropic expansion factor of 1.30 to 7.00 is adhered by means of molten tetrafluoroethylene-perfluoroolefin copolymer to a liquid-pervious reinforcing material layer which is capable of retaining its original form even when heated to a temperature of 270 to 380 C.

2. A process for making air-pervious sheets of polytetrafluoroethylene which comprises dispersing in trichlorotrifiuoroethane a polytetrafluoroethylene fibrous powder having an average fiber length of 100 to 5000 microns, an average shape factor of not less than 10, and an anisotropic expansion factor of 1.30 to 7.00, using the so obtained dispersion and forming a web, and thereafter sintering said web.

3. A process for making air-pervious sheets of polytetrafluoroethylene which comprises dispersing in carbontetrachloride a polytetrafluoroethylene fibrous powder having an average fiber length of 100 to 5000 microns, an average shape factor of not less than 10, and an anisotropic expansion factor of 1.30 to 7.00, using the so obtained dispersion and forming a web, and thereafter sintering said web.

4. A process for making air-pervious sheets predominantly of polytetrafluoroethylene which comprises dispersing in trichlorotrifiuoroethane a polytetrafluoroethylene fibrous powder having an average fiber length of 100 to 5000 microns, an average shape factor of not less than 10, and an anisotropic expansion factor of 1.30 to 7.00, and 0.2% to 50% by weight, based on said fibrous powder, of a thermoplastic resin powder, using the so obtained dispersion and forming a web, and thereafter sintering said web at a temperature above the melting point of said thermoplastic resin.

5. A process for making air-pervious sheets predominantly of polytetrafluoroethylene which comprises dispersing in carbontetrachloride a polytetrafluoroethylene fibrous powder having an average fiber length of 100 to 5000 microns, an average shape factor of not less than 10, and an anisotropic expansion factor of 1.30 to 7.00, and 0.2% to 50% by weight, based on said fibrous powder, of a thermoplastic resin powder, using the so obtained dispersion and forming a web, and thereafter sintering said web at a temperature above the melting point of said thermoplastic resin.

References Cited UNITED STATES PATENTS 5/1960 Thomas 264-127 1/1962 Hochberg l62l57 OTHER REFERENCES HOWARD R. CAINE, Primary Examiner US. Cl. X.R.

l6l18l; 162100; 264-127 

